EWs of the two C IV BAL troughs of APM 08279+5255, plotted one versus the other (black points). The red arrows connect consecutive epochs of EW pairs. The two correlation fits of BAL₁ versus BAL₂ and vice versa are shown (solid lines) along with the corresponding bisector fit ( dot-dashed line). Slope m_bis and intercept q_bis of such fit, correlation coefficient r, and corresponding null-hypothesis probability p( >r) are reported. The cross in the lower right corner represents the mean error on the data set in each direction.